Golf swing measurement system, measurement apparatus, and measurement method

ABSTRACT

The system ( 1 ) for measuring a golf swing according to the present invention is provided with at least two image pickup apparatuses that each capture an image of a golf club ( 5 ) that includes at least one head identifier attached to the head and with a measurement apparatus that measures the force acting on the head of the golf club ( 5 ) based on position information of the at least one head identifier, the position information being acquired from the images of the golf club ( 5 ) captured during a swing.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on an application No. 2012-115796 filed in Japan on May 21, 2012, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a golf swing measurement system, measurement apparatus, and measurement method.

BACKGROUND ART

During development of golf clubs, golf swings by different types of golfers have been measured and collected as data. Measuring golf swings by different types of golfers supports the development of golf clubs suitable for different types of golfers as well as the selection of a suitable golf club for each type of golfer.

In accordance with the forces acting on the golf club during a golfer's swing, the shaft of the golf club experiences deformations such as flex and torsion, and the head rotates. Deformation of the golf club shaft and rotation of the head are considered to exert an extremely large influence over whether a golfer can execute an ideal golf swing.

The forces acting on the golf club that lead to deformation of the golf club shaft and rotation of the head are mainly caused by forces exerted by the subject, such as a golfer, acting on the golf club. During the golf swing, the exertion of such forces acts on the head through the shaft, generating an inertial force, a centrifugal force, and the like at the head and possibly causing the head to rotate. Furthermore, the inertial force and the centrifugal force may end up deforming the shaft. When the shaft deforms due to torsion, the torsion may affect the rotation behavior of the head. In this way, the inertial force and the centrifugal force acting on the head during the golf swing cause the shaft to deform and also affect the rotation behavior of the head.

When analyzing a golfer's swing, it is therefore crucial to calculate the centrifugal force and inertial force acting on the head of the golf club during the swing. Conventionally, an apparatus has thus been proposed to detect at least one of velocity, acceleration, inertial force, and centrifugal force with a detection unit attached to the golfer or to the golf club (for example, see Patent Literature 1). According to the apparatus in Patent Literature 1, it is possible to detect and measure changes in the velocity during the golf swing as well as acceleration, inertial force, centrifugal force, and the like obtained as a function of velocity by using, for example, a velocity sensor, a force resistive sensor, a pressure sensor, a gravimetric sensor, or the like.

A method has also conventionally been proposed for attaching a body having an inertial sensor, constituted by at least one of a gyroscope and an accelerometer, to a measurement subject and calculating the swing velocity of the measurement subject based on a signal from the inertial sensor (for example, see Patent Literature 2). According to this method, the swing velocity can be measured to a high degree of accuracy, thereby improving reliability.

On the other hand, in recent years, non-contact measurement methods, such as image measurement, have been proposed as methods for measuring and analyzing a golf swing. For example, a behavior measurement apparatus for measuring the behavior of a golf club head has been proposed. This behavior measurement apparatus measures the behavior of the head based on an image pickup apparatus that captures images of a plurality of markers attached to the golf club, position information on the captured markers, and a 3D geometric model of the head (for example, see Patent Literature 3). Such a behavior measurement apparatus can perform highly accurate measurement of the behavior of the head. Specifically, the behavior measurement apparatus can reportedly make accurate measurements of the position and orientation of the striking face immediately before the golf ball is hit.

A golf swing measurement system has also been proposed for measuring golf swing behavior by attaching a shaft axis mark and shaft rotation mark to the golf club, attaching wrist marks, elbow marks and shoulder marks to the golfer, and using a computer to recognize the position coordinates of each mark based on captured images (for example, see Patent Literature 4). According to this sort of golf swing measurement system, it is reportedly possible to perform an accurate and detailed analysis of behavior of the wrist joints and elbow joints, which have two rotational degrees of freedom, during a swing.

CITATION LIST Patent Literature

PTL 1: JP7178210A

PTL 2: JP2011072518A

PTL 3: JP2009294048A

PTL 4: JP2005218783A

SUMMARY OF INVENTION

With the methods and techniques disclosed in Patent Literature 1 and 2, the swing velocity, acceleration, inertial force and centrifugal force are calculated using a sensor directly attached to the golf club and the like. This poses the risk of the golfer not being able to execute a golf swing as usual due to a variety of limitations caused by use of a sensor, such as wiring attached to the golf club and increased weight of the golf club. Furthermore, when using an acceleration sensor, it is necessary to adjust for the effect of the mass of the acceleration sensor included in the measurement value. Moreover, with the methods based on image measurement disclosed in Patent Literature 3 and 4, even though the limitations caused by use of a sensor do not occur, 3D measurement of velocity and acceleration is difficult. Therefore, no consideration is made for measuring the forces acting on the head during the golf swing, such as an inertial force and a centrifugal force.

The present invention has been conceived in light of the above-described circumstances, and it is an object thereof to provide a golf swing measurement system, measurement apparatus, and measurement method that can measure the forces acting on the head during a swing as usual by a golfer.

A golf swing measurement system according to an aspect of the present invention for achieving the above object is a system for measuring force acting on a head of a golf club during a swing, comprising: at least two image pickup apparatuses each configured to capture an image of the golf club, the golf club including at least one head identifier attached to the head for specification of a position on the head; and a measurement apparatus configured to measure the force acting on the head of the golf club based on position information of the at least one head identifier, the position information being acquired from the images of the golf club captured by the image pickup apparatuses during the swing.

The system according to an aspect of the present invention can measure the force acting on the head during a swing as usual by a golfer.

In the system according to an aspect of the present invention, the measurement apparatus preferably comprises an image acquisition unit configured to acquire the images of the golf club during the swing from the image pickup apparatuses; a position information acquisition unit configured to recognize the at least one head identifier in the acquired images and to acquire position information of the recognized at least one head identifier; and a measurement unit configured to measure, as the force acting on the head of the golf club, a centrifugal force and an inertial force acting on the head of the golf club based on the acquired position information.

According to this structure, it is possible easily to measure the centrifugal force and the inertial force acting on the head during a swing as usual by a golfer. In the present description, the centrifugal force is an apparent force that acts outwards, from the center of rotation, on the head of the golf club during the swing, and the inertial force is an apparent force corresponding to the acceleration acting on the head of the golf club during the swing.

In the system according to an aspect of the present invention, the measurement unit preferably measures the centrifugal force and the inertial force as the force acting on the head of the golf club by resolving the centrifugal force and the inertial force into components in a striking direction of the swing and a toe down direction.

According to this structure, it is possible to provide indices for a more detailed analysis of the swing by more accurately measuring the forces acting on the head during the swing in the striking direction and the toe down direction. In the present description, the direction perpendicular to the face surface is referred to as the striking direction. Toe down typically refers to the change in position of the head in the shaft axis direction occurring during a swing. In the present description, however, the toe down direction refers to a direction along the line of intersection of the face surface and a surface that passes through the center of gravity of the head and is orthogonal to the shaft axis.

In the system according to an aspect of the present invention, preferably the at least one head identifier is positioned for specification of an orientation and a center of gravity of the head, the image pickup apparatuses capture the images of the golf club, the golf club including the at least one head identifier and at least one shaft axis identifier positioned for specification of a shaft axis of the golf club, the position information acquisition unit acquires position information of the at least one head identifier and the at least one shaft axis identifier from the images, and the measurement unit measures the centrifugal force and the inertial force as the force acting on the head of the golf club by resolving the centrifugal force and the inertial force into the components in the striking direction of the swing and the toe down direction based on the position information of the at least one head identifier and the at least one shaft axis identifier.

According to this structure, it is possible to provide indices for a more detailed analysis of the swing by easily and more accurately measuring the force acting on the head during the swing in the striking direction and the toe down direction.

In the system according to an aspect of the present invention, the measurement unit preferably further measures, as the force acting on the golf club, torque acting on a shaft of the golf club based on the centrifugal force and the inertial force.

According to this structure, it is possible to provide indices for a more detailed analysis of torsion acting on the shaft of the golf club during the swing by measuring torque acting on the shaft during the swing. In the present description, torque refers to the force applied in the torsion direction.

A measurement apparatus according to an aspect of the present invention for achieving the above object is an apparatus for measuring force acting on a head of a golf club during a swing, wherein the apparatus measures the force acting on the head of the golf club based on position information of at least one head identifier attached to the head for specification of a position on the head, the position information being acquired from an image of the golf club captured during the swing.

The apparatus according to an aspect of the present invention can measure the force acting on the head during a swing as usual by a golfer.

A golf swing measurement method according to an aspect of the present invention for achieving the above object is a method for measuring force acting on a head of a golf club during a swing, comprising the steps of: attaching at least one head identifier to the head for specification of a position on the head; capturing an image of the golf club including the at least one head identifier; acquiring position information of the at least one head identifier from the image of the golf club captured during the swing; and determining the force acting on the head of the golf club based on the position information.

The method according to an aspect of the present invention can measure the force acting on the head during a swing as usual by a golfer.

According to the present invention, it is possible to measure the force acting on the head during a swing as usual by a golfer.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be further described below with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic view of a golf swing measurement system according to an embodiment of the present invention;

FIG. 2 is a functional block diagram schematically illustrating the structure of a golf swing measurement apparatus according to the embodiment of the present invention;

FIG. 3 is a flowchart illustrating an example of the golf swing measurement method according to the embodiment of the present invention;

FIG. 4 illustrates an example of a golf club used in the golf swing measurement system according to the embodiment of the present invention;

FIG. 5 is a diagram illustrating an example of a centrifugal force decomposition method when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 6 is another diagram illustrating an example of the centrifugal force decomposition method when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 7 is a diagram illustrating an example of an inertial force decomposition method when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 8 is another diagram illustrating an example of the inertial force decomposition method when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 9A is a diagram illustrating an example of inertial force decomposition immediately after the start of a downswing when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 9B is a diagram illustrating an example of inertial force decomposition in the middle of the downswing when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 9C is a diagram illustrating an example of inertial force decomposition immediately before impact when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 10 is a diagram illustrating an example of a decomposition method for resolving a centrifugal force into a component in a torsion direction when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 11 is another diagram illustrating an example of a decomposition method for resolving a centrifugal force into a component in the torsion direction when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 12 is a diagram illustrating an example of a method of calculating torque derived from an inertial force when measuring a swing using the golf club illustrated in FIG. 4;

FIG. 13A illustrates the results of measuring forces derived from an inertial force and a centrifugal force in a striking direction when a robot executes a swing;

FIG. 13B illustrates the results of measuring forces derived from an inertial force and a centrifugal force in a toe down direction when a robot executes a swing;

FIG. 14A illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the striking direction illustrated in FIG. 13A and a flex amount of the shaft;

FIG. 14B illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the toe down direction illustrated in FIG. 13B and the flex amount of the shaft;

FIG. 15A illustrates the results of measuring forces derived from an inertial force and a centrifugal force in the striking direction when a human executes a swing;

FIG. 15B illustrates the results of measuring forces derived from an inertial force and a centrifugal force in the toe down direction when a human executes a swing;

FIG. 16A illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the striking direction illustrated in FIG. 15A and the flex amount of the shaft;

FIG. 16B illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the toe down direction illustrated in FIG. 15B and the flex amount of the shaft;

FIG. 17A illustrates the results of measuring torque derived from an inertial force and a centrifugal force on the shaft when a robot executes a swing;

FIG. 17B illustrates the results of measuring torque derived from an inertial force and a centrifugal force on the shaft when a human executes a swing;

FIG. 18A illustrates the relationship between the sum of torque derived from the inertial force and from the centrifugal force illustrated in FIG. 17A and a torsion angle of the shaft calculated based on markers;

FIG. 18B illustrates the relationship between the sum of torque derived from the inertial force and from the centrifugal force illustrated in FIG. 17B and the torsion angle of the shaft calculated based on markers; and

FIG. 19 illustrates an example of a golf club used for measurement of torsion acting on the golf club during a swing.

DESCRIPTION OF EMBODIMENTS

The following will describe a golf swing measurement system, measurement apparatus, and measurement method according to an embodiment of the present invention in detail with reference to the drawings.

FIG. 1 is a schematic view of the golf swing measurement system according to the embodiment of the present invention. A golf swing measurement system 1 illustrated in FIG. 1 is a measurement system for measuring forces acting on the head on a golf club 5 when a golfer 4 swings the golf club 5. The measurement system 1 is provided with a first camera 2A and a second camera 2B, constituting at least two image pickup apparatuses, and a measurement apparatus 3 that measures forces acting on the head based on images of the golf club captured during the swing.

The first camera 2A and the second camera 2B capture images of a marker 6 (head identifier), which constitutes at least one identifier attached to the head for specification of a position on the head. The marker 6 is made from a material that can be distinguished from its surroundings in a captured image of the golf club 5. For example, the marker 6 is reflective tape or paint. The marker 6 is, for example, preferably positioned on the head near the base of the shaft. This makes it possible to acquire position information on the head during a swing with little effect from the flex and torsion of the golf club.

The first camera 2A and the second camera 2B are, for example, high-speed video cameras capable of highly-sensitive filming of video with abundant movement. The frame rate when the first camera 2A and the second camera 2B film the swing is, for example, 160 Hz.

The measurement apparatus 3 measures the forces acting on the head of the golf club 5 based on the position information of the marker 6 acquired from the images of the golf club 5 captured by the first camera 2A and the second camera 2B during the swing by the golfer 4. The measurement apparatus 3 is constituted by a computer provided, for example, with a Central Processing Unit (CPU) or a Digital Signal Processor (DSP).

FIG. 2 is a functional block diagram schematically illustrating the structure of the measurement apparatus 3. The measurement apparatus 3 is provided with image acquisition units 7A and 7B that acquire images, a calculation unit 10 constituted by, for example, a CPU or a DSP, a control unit 11 that controls each functional unit provided in the measurement apparatus 3, a display unit 12 that displays measurement results, and a database 13 that stores data such as measurement results and golf club specifications. The image acquisition units 7A and 7B are, for example, each constituted by an interface (I/F). Furthermore, the calculation unit 10 is provided with a position information acquisition unit 8 that acquires position information and a measurement unit 9 that measures forces acting on the head of the golf club 5. An overview of operations by each functional unit will be provided below with reference to the flowchart for the golf swing measurement method according to the embodiment of the present invention shown in FIG. 3.

FIG. 3 is a flowchart illustrating an example of the golf swing measurement method according to the embodiment of the present invention. In this explanation, the golfer 4 in FIG. 1 executes a swing with the golf club 5. First, the first camera 2A and the second camera 2B constituting at least two image pickup apparatuses capture images, during a swing, of the marker 6 attached to the head (step S01). Subsequently, via the image acquisition units 7A and 7B, the measurement apparatus 3 acquires image data for the marker 6 during the swing from the first camera 2A and the second camera 2B (step S02). The image data for the marker 6 may be video data of any format captured by the first camera 2A and the second camera 2B.

Via the position information acquisition unit 8, the measurement apparatus 3 extracts any image data from the image data captured by the first camera 2A and the second camera 2B in step S02, recognizes the marker in the extracted image data, and acquires position information on the recognized marker (step S03).

The position information acquisition unit 8 recognizes the marker attached to the golf club 5 by, for example, a circle fitting method applied, for example, to any number of frames extracted from the video data acquired from the first camera 2A and the second camera 2B. A circle fitting method is a method to extract the outline of a circular marker (a sphere in 3D space), establish a circle approximating the extracted outline, and calculate the virtual center point of the marker. Furthermore, as the position information of the marker 6, the position information acquisition unit 8 acquires 3D coordinates with a general method, such as a triangulation method. The position information acquisition unit 8 can store information on the acquired 3D coordinates in association with the golfer who executed the swing in the database 13.

Via the measurement unit 9, the measurement apparatus 3 determines forces acting on the head of the golf club 5 based on the position information acquired in step S03 (step S04). The measurement apparatus 3 can then display measurement results, as in the examples in FIGS. 14A through 18B, on the display unit 12.

The measurement unit 9 preferably measures the centrifugal force acting on the head based on the position information of the marker 6.

Specifically, the measurement unit 9 can approximate a virtual rotation center and virtual circle for the head during a swing based on the position information of the marker 6 at any time t_(n) and times before and after t_(n), i.e. t_(n−1) and t_(n+1). Note that the virtual circle can be calculated as, for example, the trajectory traversed by the position of the center of gravity of the head. The centrifugal force F_(c) acting on the head can be calculated based on the following equation, using the radius r of the virtual circle, as well as the mass m and velocity v, calculated from the position information at times t_(n), t_(n−1) and t_(n+1), of the head of the golf club 5. The centrifugal force F_(c) is an apparent force that acts outwards, from the center of rotation, on the head of the golf club during the swing.

F _(c) =mv ² /r

Furthermore, the measurement unit 9 preferably calculates the inertial force F_(i) corresponding to acceleration obtained by differentiating the absolute value of the above velocity v. Letting the derivative of the absolute value of the velocity of the head of the golf club 5 be a and the mass of the head be m, the inertial force F_(i) is calculated with the following equation.

F _(i) =−ma

The inertial force F_(i) referred to in the present description is an apparent force corresponding to the acceleration acting on the head at each point in time.

By thus performing non-contact measurement of a golf swing, the golf swing measurement system 1 according to the present embodiment can measure the forces acting on the head during a golf swing as usual by a golfer. Furthermore, as the forces acting on the head of the golf club, the golf swing measurement system according to the present embodiment can measure the centrifugal force and the inertial force acting on the head. Moreover, performing measurements without use of an acceleration sensor eliminates the effect of the mass of the acceleration sensor on the measurement value, thereby offering the advantage of not requiring calculation to adjust for the effect of the mass of the acceleration sensor.

The forces acting in the striking direction of the swing and the toe down direction greatly affect the direction and distance traveled by the golf ball, the travel velocity, and the like. Therefore, as indices for assessing the swing, it is also extremely important to assess forces acting in the striking direction of the swing and the toe down direction. Furthermore, toe down of the head when the golf club is swung has been considered to be caused by centrifugal force generated during the swing, but the causal relationship has not been made clear. The striking direction of the swing refers to the direction in which the ball is struck by the swing and is determined by the orientation of the face surface of the head at the time of impact. Therefore, in the present description, the direction perpendicular to the face surface is referred to as the striking direction. Toe down typically refers to the change in position of the head in the shaft axis direction occurring during a swing. In the present description, however, the toe down direction refers to a direction along the line of intersection of the face surface and a surface that passes through the center of gravity of the head and is orthogonal to the shaft axis.

Furthermore, via the measurement unit 9, the golf swing measurement system 1 according to the present embodiment preferably measures the centrifugal force F_(c) and the inertial force F_(i) as the forces acting on the head of the golf club by resolving these forces into components in the striking direction of the swing and the toe down direction. The method for resolving the forces acting on the head of the golf club into the components in the striking direction of the swing and the toe down direction will be described with reference to FIGS. 4 through 8.

FIG. 4 illustrates an example of a golf club used in the golf swing measurement system according to the present embodiment. A golf club 18 has a grip 14, a shaft 15, and a head 16. The golf club 18 preferably has shaft axis markers M1 and M2 (shaft axis identifiers) positioned to allow for specification of the axis of the shaft 15 and head markers M3 and M4 (head identifiers) positioned to allow for specification of the orientation and the center of gravity of the head. The shaft axis markers M1 and M2 are positioned to allow for specification of the axis of the shaft 15. For example, the shaft axis markers M1 and M2 are positioned with their center along the shaft axis. The shaft 15 flexes during a swing, but since the shaft axis markers M1 and M2 are positioned near the grip 14, which flexes relatively less, the shaft axis calculated based on the shaft axis markers M1 and M2 corresponds to the axis of the shaft 15 when the shaft 15 does not flex at all during a swing.

The head markers M3 and M4 are preferably positioned on the head 16 in a configuration such that information on the orientation of the face surface of the head 16 and on the position of the center of gravity of the head 16 can be obtained. Accordingly, the configuration of the head markers is not limited to the configuration shown in the drawings. For example, it is of course possible to use only one head marker in the shape of a line parallel to the face surface. The measurement system 1 obtains information on the position of the center of gravity during a swing by acquiring information on the position of the center of gravity of the head 16 from the golf club specifications and determining the distance to the head markers M3 and M4 and the axis of the shaft 15 as well as the relative positions thereof with respect to the center of gravity. Note that while the trajectory traversed by the position of the center of gravity has been assumed to be a virtual circle, it is of course possible to adopt a simpler method that calculates the forces acting on the head 16 by calculating the trajectory traversed by the head marker as the virtual circle.

FIG. 5 is a diagram illustrating an example of a centrifugal force decomposition method when measuring a swing using the golf club illustrated in FIG. 4, the swing being viewed from the front. For clarity, the shaft 15 and the grip 14 are not illustrated but rather are represented by a shaft axis 15A. The centrifugal force F_(c) is an apparent force that acts outwards, from the above-described virtual rotation center RC, on the center of gravity G of the head 16. The magnitude of the centrifugal force F_(c) can be represented as F_(c)=mv²/r (where m is the mass of the head, v is the velocity of the head, and r is the radius of the virtual circle). In order to resolve the centrifugal force F_(c) in the striking direction of the swing and the toe down direction, the measurement unit 9 first resolves the centrifugal force F_(c) into a force F₁ in a direction perpendicular to the shaft axis 15A. This decomposition is performed, for example, with the following equation.

F ₁ =F _(c) sinθ₁

Here, the angle θ₁ is the angle between the shaft axis 15A and a line connecting the virtual rotation center RC with the center of gravity G of the head 16.

FIG. 6 is a top view illustrating an example of a centrifugal force decomposition method when measuring a swing using the golf club illustrated in FIG. 4. In FIG. 6, a line parallel to the face surface of the head 16 connecting the head markers M3 and M4 is labeled FL, and a line parallel to the line FL and traversing the center of gravity G is labeled FL_(p). For clarity, the head markers M3 and M4 and the shaft 15 are illustrated with their respective centers aligned with the face surface of the head 16. The force F₁ calculated as described above is a force in the normal direction from the center of gravity G towards the shaft axis 15A, as illustrated in FIG. 6. Based on the center of gravity angle θ_(G), the force F₁ is resolved by the following equation into a striking direction centrifugal force F_(cf) and a toe down direction centrifugal force F_(ct) centered on the center of gravity G.

F _(cf) =F ₁ sinθ_(G)

F _(ct) =F ₁ cosθ_(G)

Note that the center of gravity angle θ_(G) is the angle between the face surface of the head and a line connecting the position of the center of gravity with the center of the position of attachment of the shaft 15. The center of gravity angle θ_(G) may also be acquired from the specifications for the golf club 18 or the like. Furthermore, the center of gravity angle θ_(G) can be derived from calculating the angle between a line parallel to the face surface of the head acquired from the head markers M3 and M4 and the line connecting the position of the center of gravity with the center of the position of attachment of the shaft 15. In this way, the golf swing measurement system 1 according to the present embodiment can calculate the striking direction centrifugal force and the toe down direction centrifugal force acting on the golf club 18 during a swing.

FIGS. 7 and 8 are diagrams illustrating an example of a decomposition method for an inertial force F_(i) when measuring a swing using the golf club illustrated in FIG. 4. In these figures, the angle θ₂ is the angle between a line 15P parallel to the shaft axis 15A and a tangent a to the virtual circle 19, which approximates the trajectory of the center of gravity G during the swing. The measurement unit 9 in FIG. 2 differentiates the absolute value of the velocity during the swing to calculate the magnitude of the acceleration a in a direction matching the tangent to the virtual circle 19. FIG. 7 illustrates the inertial force F_(i), which is an apparent force that acts in the opposite direction of the acceleration a. Furthermore, by the following equation, the measurement unit 9 measures a force F₂ that traverses the center of gravity G in a direction perpendicular to the shaft axis 15A.

F ₂ =F _(i) sinθ₂

As illustrated in FIG. 8, the measurement unit 9 resolves the force F₂ into a toe down direction inertial force F_(it) and a striking direction inertial force F_(if) centered on the center of gravity G using the equations below. Here, the angle θ is the angle between the face surface of the head and the inertial force vector. In FIG. 8, as in FIG. 6, a line parallel to the face surface of the head 16 connecting the head markers M3 and M4 is labeled FL, and a line parallel to the line FL and traversing the center of gravity G is labeled FL_(p).

F _(if) =F ₂ sinθ

F _(it) =F ₂ cosθ

As described with reference to FIGS. 7 and 8, the golf swing measurement system according to the present embodiment can resolve, in the striking direction and in the toe down direction, the inertial force acting on the head 16 of the golf club 5 during a swing. As a result, the measurement system 1 can provide indices for a more detailed analysis of the swing. An example of resolving the inertial force at three points in time during a golf swing will now be described with reference to FIGS. 9A through 9C. The three points in time during the golf swing are (1) immediately after the start of the downswing, in which the golfer 4 swings the golf club 5 down; (2) the middle of the downswing; and (3) immediately before impact of the head 16 with the golf ball. At the point in time (1) immediately after the start of the downswing, the face of the head 16 is open and is facing ahead of the golfer 4. At the point in time (2) at the middle of the downswing, the face is closed and is slanted with respect to the front of the golfer 4. At the point in time (3) immediately before impact, the face is closed and is facing sideways with respect to the front of the golfer 4.

FIG. 9A is a top view illustrating an example of inertial force decomposition immediately after the start of a downswing. Immediately after the start of the downswing, the face surface of the head 16 is open towards the front of the golfer 4. Since the golfer 4 swings the golf club 5 down with the head 16 in this state, the head 16 moves in the direction of the shaft axis 15A.

Therefore, the acceleration ah acts from the center of gravity G towards the left in the figure, and the inertial force F_(i) is a force in the opposite direction. Using the same equations as described with reference to FIG. 8, the measurement unit 9 resolves the force F₂ into a toe down direction inertial force F_(it) and a striking direction inertial force F_(if) centered on the center of gravity G.

FIG. 9B is a diagram illustrating an example of inertial force decomposition in the middle of the downswing. The face surface of the head 16 gradually closes in the middle of the downswing so as to be slanted with respect to the front of the golfer 4. Therefore, the acceleration ah acts in the direction shown in the figure, and the inertial force F_(i) is a force in the opposite direction. Using the same equations as described with reference to FIG. 8, the measurement unit 9 resolves the force F₂ into a toe down direction inertial force F_(it) and a striking direction inertial force F_(if) centered on the center of gravity G.

FIG. 9C is a diagram illustrating an example of inertial force decomposition immediately before impact. The face surface of the head 16 is closed immediately before impact and faces sideways with respect to the front of the golfer 4. Therefore, the acceleration ah acts in the direction shown in the figure, and the inertial force F_(i) is a force in the opposite direction. Using the same equations as described with reference to FIG. 8, the measurement unit 9 resolves the force F₂ into a toe down direction inertial force F_(it) and a striking direction inertial force F_(if) centered on the center of gravity G.

As described with reference to FIGS. 4 through 9, the golf swing measurement system 1 according to the present embodiment can resolve, in the striking direction and the toe down direction, the centrifugal force and the inertial force acting on the golf club during a swing. In this way, among the forces acting on the head of the golf club, the force contributing in the striking direction and the force contributing in the toe down direction are calculated, thus permitting a more detailed analysis of the swing.

Furthermore, the golf swing measurement system 1 according to the present embodiment preferably also measures the torque contributing to torsion of the shaft as a force acting on the golf club by measuring the torque from each of the centrifugal force and the inertial force acting on the golf club during the swing. This measurement will be described with reference to FIGS. 10 through 12.

FIG. 10 is a diagram illustrating an example of a decomposition method for resolving a centrifugal force into a component in a torsion direction when measuring a swing using the golf club illustrated in FIG. 4. The torsion direction refers to the direction in which the head rotates around the shaft axis of the golf club and is a rotational direction either forwards or backwards with respect to the direction of travel. In the present description, the force applied in the torsion direction is referred to as torque. FIG. 10 is a front view of the golf club 5. A line parallel to the shaft axis 15A and traversing the center of gravity G of the head 16 is labeled 15P. The centrifugal force F_(c) acting on the head 16 during the swing acts on the center of gravity G of the head 16 in a direction away from the above-described virtual rotation center RC. Therefore, in order first to calculate the centrifugal force in the torsion direction, the measurement unit 9 needs to calculate a component F_(cg) of the centrifugal force F_(c) along the line 15P by the following equation, using the angle θ₁ between the shaft axis 15A of the shaft 15 of the golf club 18 and a line connecting the virtual rotation center RC with the center of gravity G of the head 16.

F _(cg) =F _(c) cosθ₁

The measurement unit 9 then uses the component F_(cg) of the centrifugal force F_(c) along the line 15P as well as an angle θ_(SG), which is the angle between the shaft axis 15A and a line connecting the position at which the shaft axis 15A meets the head 16 (referred to below as the “contact point”) with the center of gravity G of the head 16, to calculate a force F_(cgf) using the following equation. The force F_(cgf) is a force acting on the center of gravity G with the contact point as a point of support and is a decomposition, from the component F_(cg) of the centrifugal force F_(c), of a force in a direction approximately perpendicular to the face of the head 16.

F _(cgf) =F _(cg)*sinθ_(SG)*cosθ_(SG)

Here, sinθ_(SG) and cosθ_(SG) can be derived from the value of tanθ_(SG). The value of tanθ_(SG) can be calculated by the following equation based on the face height α, the center of gravity height β, and the center of gravity depth γ.

tanθ_(SG)=center of gravity depth γ/(face height α−center of gravity height β)

The face height α refers to the height from the bottom of the golf club to the upper edge of the face. The center of gravity height β refers to the height from the bottom of the golf club to the center of gravity in the direction of the shaft axis 15A. The center of gravity depth y refers to the distance from the center of gravity of the golf club to the face surface in a direction perpendicular to the shaft axis 15A. Note that the face height α, center of gravity height β, and center of gravity depth γ for a variety of golf club heads can be obtained from the specifications for the golf clubs.

As illustrated in FIG. 11, the measurement unit 9 calculates a torsion direction centrifugal force F_(ctr) by the following equation, using the above-described force F_(cgf) and the center of gravity angle θ_(G) between the face surface of the head and a line connecting the position of the center of gravity with the center of the position of attachment of the shaft 15. The torsion direction centrifugal force F_(ctr) is a centrifugal force in the torsion direction derived from the centrifugal force acting on the center of gravity G during the swing.

F _(ctr) =F _(cgf)*cosθ_(G)

Finally, with the following equation, the measurement unit 9 calculates the torque T_(c) around the shaft axis 15A derived from the centrifugal force by multiplying the torsion direction centrifugal force F_(ctr) by the center of gravity distance L.

T _(c) =F _(ctr) *L

Note that the center of gravity distance L refers to the distance from the center of gravity G of the head 16 to the shaft axis 15A and can be obtained from the specifications for the golf club.

FIG. 12 is a diagram illustrating an example of a method of calculating the torque T_(i) derived from an inertial force. The measurement unit 9 in FIG. 2 calculates the inertial force F_(i) as F_(i)=−ma, where a is the derivative of the absolute value of the velocity v of the head 16 and m is the mass of the head 16. The measurement unit 9 then uses the center of gravity angle θ_(G) and an angle θ_(FL) between a tangent to the trajectory of the head 16 and the line FL parallel to the face surface to calculate a torsion direction inertial force F_(itc,) around the shaft axis 15A, that forms a 90° angle with respect to a line connecting the center of gravity G and the shaft axis 15A and that acts backward with respect to the direction of travel of the head 16.

F _(itc) =F _(i)*cos(90°−θ_(FL)+θ_(G))

With the following equation, the measurement unit 9 then calculates the torque T_(i) around the shaft axis 15A derived from the inertial force by multiplying the torsion direction inertial force F_(itr) by the center of gravity distance L.

T _(i) =F _(itr) *L

Note that the center of gravity distance L refers to the distance from the center of gravity G of the head 16 to the shaft axis 15A and can be obtained from the specifications for the golf club.

In this way, the golf swing measurement system 1 according to the present embodiment can calculate the torque derived from the centrifugal force F_(c) and from the inertial force F_(i). It is considered that the torque T_(c) derived from the centrifugal force causes torsion in the direction of travel, whereas the torque T_(i) derived from the inertial force causes torsion in the direction opposite the direction of travel. Accordingly, taking sign into consideration, it is thought that if the sum of the torque (T_(c)+T_(i)) is a positive value, the shaft 15 of the golf club 18 is twisted in the direction of travel. Conversely, if the sum of the torque (T_(c)+T_(i)) is a negative value, torsion occurs in the direction opposite the direction of travel. Accordingly, by summing the calculated values of torque, the golf swing measurement system 1 can analyze the torsion acting on the shaft 15 of the golf club 18 in greater detail.

Measurement results by the golf swing measurement system 1 according to the present embodiment will now be described with reference to FIGS. 13A through 16B. Here, measurement was performed for a swing using a golf club having an additional marker on the head 16 near the base of the shaft 15 of the golf club 18 illustrated in FIG. 4. FIG. 13A illustrates the results of measuring forces derived from an inertial force and a centrifugal force in the striking direction when a robot executes a swing. FIG. 13B illustrates the results of measuring forces derived from an inertial force and a centrifugal force in the toe down direction when a robot executes a swing. The vertical axis represents the magnitude of the force (N), and the horizontal axis represents time (s). Time 0 is the time of impact.

FIG. 14A illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the striking direction illustrated in FIG. 13A and the flex amount of the shaft 15. FIG. 14B illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the toe down direction illustrated in FIG. 13B and the flex amount of the shaft 15. The flex amount of the shaft 15 can be calculated as the distance between the above-described additional marker near the base of the shaft 15 and the shaft axis determined by the shaft axis markers M1 and M2 illustrated in FIG. 4. As clearly shown in FIGS. 14A and 14B, the flex amount of the shaft as measured with the additional marker and the sum of the centrifugal force and the inertial force exhibit a similar tendency of change over time. Based on this relationship, the golf swing measurement system 1 according to the present embodiment can quantitatively measure the change over time in the magnitude of the applied force with respect to the flex of the shaft during a swing.

FIG. 15A illustrates the results of measuring forces derived from an inertial force and a centrifugal force in the striking direction when a human executes a swing. FIG. 15B illustrates the results of measuring forces derived from an inertial force and a centrifugal force in the toe down direction when a human executes a swing. Furthermore, FIG. 16A illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the striking direction illustrated in FIG. 15A and the flex amount of the shaft. FIG. 16B illustrates the relationship between the sum of forces derived from the inertial force and the centrifugal force in the toe down direction illustrated in FIG. 15B and the flex amount of the shaft. For a human as well, it is clear that the flex amount of the shaft and the sum of the centrifugal force and the inertial force exhibit a similar tendency of change over time.

Another example of measurement results by the golf swing measurement system 1 according to the present embodiment will now be described with reference to FIGS. 17A through 18B. Here, measurement was performed for a swing using a golf club 18A illustrated in FIG. 19. The golf club 18A is the golf club 18 illustrated in FIG. 4 with a marker M5 on a mounting 20 at the upper portion of the shaft 15. In order to verify the torque acting on the golf club during a swing measured by the measurement system 1 according to the present embodiment, the torsion acting on the golf club during a swing was calculated based on the relative positions of the head markers M3 and M4 on the head 16 and the marker M5.

FIG. 17A illustrates the results of measuring torque derived from an inertial force and a centrifugal force on the shaft 15 when a robot executes a swing. FIG. 17B illustrates the results of measuring torque derived from an inertial force and a centrifugal force on the shaft 15 when a human executes a swing. With the above-described method, the torque derived from each of the centrifugal force and the inertial force on the shaft 15 was measured, and the sum of the values of the torque was calculated. FIGS. 18A and 18B illustrate the relationship between the sum of torque derived from the inertial force and from the centrifugal force illustrated in FIGS. 17A and 17B and the torsion angle of the shaft calculated based on the above-described markers. As shown by FIGS. 18A and 18B, the sum of the calculated torque values and the torsion angle of the golf club exhibit a similar tendency of change.

It is to be noted that many modifications and substitutions within the scope and spirit of the present invention will be apparent to a person of ordinary skill in the art. Accordingly, the present invention is not limited to the above embodiment but rather may be modified or altered in a variety of ways without deviating from the scope of the present invention.

REFERENCE SIGNS LIST

1: Measurement system

2A, 2B: Camera (image pickup apparatus)

3: Measurement apparatus

4: Golfer

5, 18: Golf club

6: Marker (head identifier)

7A, 7B: Image acquisition unit

8: Position information acquisition unit

9: Measurement unit

10: Calculation unit

11: Control unit 

1. A system for measuring force acting on a head of a golf club during a swing, comprising: at least two image pickup apparatuses each configured to capture an image of the golf club, the golf club including at least one head identifier attached to the head for specification of a position on the head; and a measurement apparatus configured to measure the force acting on the head of the golf club based on position information of the at least one head identifier, the position information being acquired from the images of the golf club captured by the image pickup apparatuses during the swing.
 2. The system according to claim 1, wherein the measurement apparatus comprises: an image acquisition unit configured to acquire the images of the golf club during the swing from the image pickup apparatuses; a position information acquisition unit configured to recognize the at least one head identifier in the acquired images and to acquire position information of the recognized at least one head identifier; and a measurement unit configured to measure, as the force acting on the head of the golf club, a centrifugal force and an inertial force acting on the head of the golf club based on the acquired position information.
 3. The system according to claim 2, wherein the measurement unit measures the centrifugal force and the inertial force as the force acting on the head of the golf club by resolving the centrifugal force and the inertial force into components in a striking direction of the swing and a toe down direction.
 4. The system according to claim 3, wherein the at least one head identifier is positioned for specification of an orientation and a center of gravity of the head, the image pickup apparatuses capture the images of the golf club, the golf club including the at least one head identifier and at least one shaft axis identifier positioned for specification of a shaft axis of the golf club, the position information acquisition unit acquires position information of the at least one head identifier and the at least one shaft axis identifier from the images, and the measurement unit measures the centrifugal force and the inertial force as the force acting on the head of the golf club by resolving the centrifugal force and the inertial force into the components in the striking direction of the swing and the toe down direction based on the position information of the at least one head identifier and the at least one shaft axis identifier.
 5. The system according to claim 4, wherein the measurement unit further measures, as the force acting on the golf club, torque acting on a shaft of the golf club based on the centrifugal force and the inertial force.
 6. An apparatus for measuring force acting on a head of a golf club during a swing, wherein the apparatus measures the force acting on the head of the golf club based on position information of at least one head identifier attached to the head for specification of a position on the head, the position information being acquired from an image of the golf club captured during the swing.
 7. A method for measuring force acting on a head of a golf club during a swing, comprising the steps of: attaching at least one head identifier to the head for specification of a position on the head; capturing an image of the golf club including the at least one head identifier; acquiring position information of the at least one head identifier from the image of the golf club captured during the swing; and determining the force acting on the head of the golf club based on the position information. 